VI. Resources for Teaching Science

Analogy - Cell

The cell is the basic unit of living systems. Although it is relatively easy to visualize the components of cells, it is difficult to conceptualize how these components function together to sustain life within the cell. To develop an understanding of the functional processes that take place in a cell, it is helpful to draw an analogy to an automobile factory.

Target concept: The dictionary defines a cell as “the smallest independently functioning unit in the structure of an organism, usually consisting of one or more nuclei surrounded by cytoplasm and enclosed in a membrane”. Although this is a relatively accurate description, it is not an intuitive description that students can readily relate to. Because cells are microscopic, difficult to visualize in three dimensions, and exceedingly complex in function, most teachers draw analogies to help students grasp their significance. It is common to hear teachers draw analogies between cells and cities, schools, and factories. In this example we are illustrating an analogy between a cell and a factory, but it should be understood that others can be equally well developed.

Analogue concept: An analogy is helpful only when people are familiar with the analogue concept, in this case a factory. Relatively few students have visited factories, and so it is helpful to review the organization of a factory as the analogy is developed. A factory is a facility where goods are manufactured for export. A factory consumes raw materials and energy in an effort to sustain its workers and provide resources to others. This is analogous to the functioning of a cell (figure 10.5) as seen in the following mapping.

Relevant features / Mapping:

cell « factory: A cell (in a multicellular organism) is an identifiable unit that is part of a larger organism just as a factory is an identifiable unit that is part of society. A cell uses material and energy resources and produces products just as a factory does.

cell products « factory products: Cells do not work in isolation but provide and receive resources from other cells or from the surrounding environment. Factories produce products so that they may be sold and earn money for the corporation. Similarly, cells produce products that may be used by surrounding cells, and in turn receive resources from surrounding cells. For example, cells in the Beta Islets of Langerhans in the pancreas produce insulin that is used by surrounding cells. Simultaneously, these same cells receive oxygen and nutrients provided by the red blood cells and plasma that flow in their neighborhood.

organelle membranes « walls: In factories, walls are used to separate regions with different functions. For example, the paint room is separated from the upholstery shop so each can carry on its functions without interference from the other.

nucleus « headquarters: The nucleus of a cell controls the operations of a cell. In a similar manner, factory headquarters controls the operations of a factory. The nucleus is separated from the rest of the cell by a nuclear membrane that allows it to function without interruption from surrounding organelles, just as the office walls of the headquarters allow planners and managers to direct the operations of the factory without being distracted by surrounding operations.

nuclear pore « doors: Communication is essential to the proper functioning of a factory. Doors allow people from different departments to visit and communicate. In a similar manner, nuclear pores allow for information and resources to flow between the nucleus and the cell it manages.

DNA/chromosome « plans: Each factory has plans that govern the production and development of their products, as well as plans that govern the day-to-day operation of the factory. DNA is analogous to such plans, providing the code not only for all cell products, but also the proteins that govern daily operations within the cell.

smooth endoplasmic reticulum « hallways: Factories have hallways through which information from the office travels to the workers in all departments. Similarly, the cell has smooth endoplasmic reticulum through which it is believed that messenger RNA travels from the nucleus to places where it is decoded.

ribosome « worker: Factory workers “translate” instructions from headquarters into products. In an analogous fashion, ribosomes are the site where messenger RNA is translated into proteins.

rough endoplasmic reticulum « assembly line: Workers gather in regions of the factory where assembly takes place. Similarly, ribosomes are positioned on rough endoplasmic reticulum where the proteins are assembled.

protein « product: Factories produce products for internal and external use. Similarly, cells produce proteins for internal use and for export. Proteins result when DNA code has been transcribed into RNA and translated into polypeptide chains. Similarly, factory products result when plans from the office are copied and distributed to workers who combine various components to assemble a product.

cytoplasm « stock room: A factory needs a stock room from which parts can be taken for use on the assembly line. Similarly, resources are distributed through the cytoplasm until they are used by surrounding organelles.

m-RNA«photocopy: Although the master plans of a company may be protected in the factory headquarters, individual plans may be photocopied from these plans and distributed to workers as needed. m-RNA is like a photocopy of DNA that accurately transmits data from the nucleus to the ribosomes where it is translated into useful proteins.

t-RNA « stockroom helpers: Stockroom helpers bring components to the assembly line where they can be combined into products. Similarly, t-RNA brings amino acids from the cytoplasm to the ribosomes where assemblage of proteins may take place.

mitochondrion « powerhouse: A powerhouse converts energy from one form to another. For example, the powerhouse at a hydroelectric dam converts the kinetic energy of falling water into electrical energy for distribution to homes and businesses. In a similar manner, mitochondria convert energy from the bonds in glucose to the phosphate bonds of ATP.

ATP « electricity: Electricity is a flexible energy source that can be easily distributed to homes and businesses. Similarly, ATP is a flexible energy source that is used to power the growth, movement and metabolism of the cell.

Golgi apparatus « warehouse: Products from a factory are generally stored in a warehouse before exporting. The Golgi apparatus may serve as a warehouse where proteins are stored prior to export.

Limitations: This analogy should not be extended to cell division. Whereas industries grow by the addition of new factories, cells grow by the splitting and growth of existing cells. Thus, students should not try to find analogues for such things as asters, centrioles, spindle fibers, etc.

Student activity: Working in a team, develop a new analogy for a cell. You may wish to analogize a cell to a city, your school, your home, or another entity of your choice.